The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods of recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale nor does it contain oil. It is a semdimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen", which upon heating decomposes to produce liquid and gaseous products including hydrocarbon products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing oil shale which involve either first mining the kerogen bearing shale and processing the shale on the surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact since the spent shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes. According to both of these approaches, oil shale is retorted by heating the oil shale to a sufficient temperature to decompose kerogen and produce shale oil which drains from the rock. The retorted shale after kerogen decomposition contains substantial amounts of residual carbonaceous material which can be burned to supply heat for retorting.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, one of which is U.S. Pat. No. 3,661,423, issued May 9, 1972, to Donald E. Garrett, assigned to the assignee of this application and incorporated herein by reference. This patent describes in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale by fragmenting such formation to form a cavity containing a stationary, fragmented permeable body or mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort. The cavity has bottom, top, and side boundaries of unfragmented formation. Hot retorting gases are passed through the in situ oil shale retort to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing "retorted oil shale".
The retort can be filled to the top with the fragmented permeable mass of particles, which is also known as a rubble pile. An upper portion of the fragmented permeable mass is ignited and an oxygen supplying gas such as air is forced downwardly through the fragmented permeable mass as a combustion zone feed for combustion of carbonaceous material in the shale. Initially some of the shale oil may be burned, but as retorting progresses, much of the combustion is of residual carbonaceous material remaining in retorted oil shale. This reduces the oxygen content of the oxygen supplying gas and the resultant gas passing downwardly through the retort below the combustion zone is essentially inert, i.e., is substantially free of free oxygen. This inert gas transfers heat downwardly and results in retorting of the oil shale in a retorting zone below the combustion zone without appreciable combustion of shale oil. It will be recognized that the rate of progression of the combustion zone is quite slow and is ordinarily in the order of only a few feet per day.
An operating retort has a combustion zone advancing slowly downwardly through the fragmented mass. This combustion zone is not a thin layer but ordinarily has appreciable thickness due to gradual consumption of oxygen in the downwardly flowing gas and inherent variations in particle size of the oil shale. The primary combustion zone is the portion of the retort where the greater part of the oxygen in the combustion feed that reacts with residual carbonaceous material in retorted oil shale is consumed. Below the primary combustion zone during normal operation is a retorting zone which is heated to a temperature sufficient to decompose the kerogen to produce liquid and gaseous products including liquid and gaseous hydrocarbon products. Above the primary combustion zone is ordinarily a zone of hot combusted oil shale.
It is found that the yield of liquid and gaseous hydrocarbon products from oil shale tends to be maximized when the primary combustion zone extends across the entire fragmented permeable mass and moves through the retort as a substantially planar wave. When the primary combustion zone is not planar, the yield of liquid and gaseous hydrocarbon products from the oil shale tends to be minimized. This minimizing occurs firstly because the oil shale in the upper corners and/or near the side edges of the retort are bypassed by the primary combustion zone and retorting zone and secondly because some of the shale oil produced by one portion of the primary combustion zone can be consumed by oxidation in another portion. In addition, when the primary combustion zone is not planar, excessive cracking of hydrocarbon products produced in the retorting zone can result.
Establishment of a combustion zone in the retort can be effected according to the method described in U.S. Pat. No. 3,990,835, issued Nov. 9, 1976 and U.S. Pat. No. 3,952,801 issued Apr. 27, 1976, both of which were issued to Robert S. Burton III and assigned to the assignee of this application. Both of these patents ae incorporated herein by this reference. U.S. Pat. No. 3,952,801 describes a technique for establishing a combustion zone in a retort by igniting the top of a fragmented permeable mass in the retort. According to this technique, a hole is bored to the top of the fragmented permeable mass and a burner is lowered through the bore hole to the oil shale to be ignited. A mixture of a combustible fuel such as LPG (liquefied petroleum gas) and gas containing oxygen, such as air, is burned in the burner and the resultant flame is directed downwardly towards the fragmented permeable mass. The burning is conducted until a substantial portion of the oil shale has been heated above the self-ignition temperature of carbonaceous material in the oil shale so combustion of oil shale in the fragmented mass is self-sustaining. Then introduction of fuel is terminated, the burner is withdrawn from the retort through the hole, and oxygen supplying gas is introduced to the retort to advance the combustion zone through the retort.
An in situ oil shale retort may have a substantial lateral extent; for example, it may be square with a width of 100 feet or more. Ignition of the top portion of the fragmented permeable mass in a completely filled retort requires access which is ordinarily obtained by forming a conduit through the overlying unfragmented rock. In a relatively smaller retort a single central conduit can be used.
In a larger retort a number of conduits to various top portions of the retort may be preferred. These conduits supply combustion air or other oxygen supplying gas during normal operation of the retort, and their openings into the retort are also used to locate points of ignition for starting the formation of a retorting zone. Since the ignition points are isolated, the top of the retort is inherently nonuniformly ignited.
For purposes of exposition, a single ignition point in the center of a retort can be assumed. A retort with a number of separate ignition points can be considered as a plurality of adjacent smaller retorts, each with a single ignition point. Ignition is obtained by burning a combustible gas with air or other oxygen supplying gas and impinging the flame on the fragmented permeable mass at the opening of the conduit. This heating can be conducted for a substantial period of time so that a sufficient volume of oil shale is heated to sustain combustion after the initial burning is stopped and an oxygen supplying gas such as air is forced down the conduit. The combustion zone that is formed around the ignition point tends to progress downwardly and outwardly. It is driven downwardly by the gas flowing through the retort and progresses laterally primarily by conduction and radiation which are much slower. Substantial unburned portions may be left in the upper corners and/or near the side edges of the retort. The self ignition temperature of the carbonaceous material in oil shale can vary with various conditions such as total gas pressure and the partial pressure of oxygen in the retort, and may be as low as 500.degree. F., although 750.degree. F. is usually considered a minimum. In operation of an in situ retort it is preferred to consider 900.degree. F. as the self ignition temperature since this is a good ignition value. Temperatures in the combustion zone of a retort may be 1200.degree. F. or more.
A combustion zone which is non planar and/or which has advanced through the in situ oil shale retort without the desired lateral spreading will result in a yield of liquid and gaseous hydrocarbon products which is not maximized. It is therefore desirable to provide a technique for establishing a combustion zone in an in situ oil shale retort where the combustion zone is flat and uniformly transverse to its direction of advancement and extends laterally to the boundaries of the retort.